Injury to vital organs, such as, for example, the heart, brain, lungs, kidneys, gastrointestinal tract, or liver, has serious and even life-threatening consequences as does damage to cells and tissues which include, for example, cornea, retina, bone, heart valves, tendons, ligaments, cartilage, vasculature, skin, bone marrow, blood cells, stem cells, and other tissues and cells derived from the body. Following injurious events, such as ischemia/hypoxia (e.g., a consequence of a heart attack, a stroke, tachycardia, atherosclerosis, hypotension (e.g. in septic shock, heart failure), thromboembolism (e.g. pulmonary embolism), outside compression of a blood vessel (e.g. by a tumor), foreign bodies in the circulation (e.g. amniotic fluid in amniotic fluid embolism), sickle cell disease, hemorrhage, or rupture of a vessel (e.g. aortic aneurysm rupture), or organ transplantation) cellular damage ensues. For example, following a stroke, the normal response of the surrounding brain is to mount a cellular response that includes formation of reactive astrocytes that are believed to be important to “contain” and “clean-up” the injury site. Swelling of neural cells is part of the cytotoxic or cell swelling response that characterizes brain damage in cerebral ischemia and traumatic brain injury, and is a major cause of morbidity and mortality. See, Staub et al., 1993; Kimelberg et al., 1995. A number of mediators have been identified that initiate swelling of neural cells, including elevation of extracellular K+, acidosis, release of neurotransmitters and free fatty acids. See, Kempski et al., 1991; Rutledge and Kimelberg, 1996; Mongin et al., 1999. Cytotoxic edema is a well-recognized phenomenon clinically that causes brain swelling, which worsens outcome and increases morbidity and mortality in brain injury and stroke.
Secondary Injury—Progressive Hemorrhagic Necrosis (PHN)
Delayed injury is an important phenomenon and represents a potential therapeutic target for ischemia/hypoxia associated injuries. The concept of delayed or secondary injury following, for example, ischemia/hypoxia, arises from the observation that the volume of injured tissue increases with time after injury, i.e., the lesion itself expands and evolves over time. Whereas primary injured tissues are irrevocably damaged from the very beginning, for example, following ischemia/hypoxia, tissues that are destined to become “secondarily” injured are considered to be potentially salvageable. An example of secondary injury in spinal cord injury (SCI) has been described and reviewed in a paper by Tator (1991), as well as in more recent reviews (Kwon et al., 2004), wherein the overall concept of secondary injury is validated. Older observations based on histological studies that gave rise to the concept of lesion-evolution have been confirmed with non-invasive MRI (Bilgen et al., 2000; Ohta et al., 1999; Sasaki et al., 1978; Weirich et al., 1990).
Mechanisms of delayed hemorrhage and PHN
Tator and Koyanagi (1997) expressed the view that obstruction of small intramedullary vessels by the initial mechanical stress or secondary injury may be responsible for PHN. Kawata and colleagues (1993) attributed the progressive changes to leukocyte infiltration around the injured area leading to plugging of capillaries. Most importantly, damage to the endothelium of spinal cord capillaries and postcapillary venules has been regarded as a major factor in the pathogenesis of PHN (Griffiths et al., 1978; Kapadia, 1984; Nelson et al., 1977). Endothelial dysfunction and damage has also been attributed to myocardial ischemic events (Verma et al. Circulation. 2002; 105:2332). The notion that the endothelium is involved in ischemia/hypoxia injury is essentially certain and represents a viable therapeutic target for protection against ischemia/hypoxia associated injuries. However, no molecular mechanism for progressive dysfunction of endothelium has heretofore been identified.
“Hemorrhagic conversion” is a term familiar in the ischemia/hypoxia injury literature. Hemorrhagic conversion describes the process of conversion from a bland infarct into a hemorrhagic infarct, and is typically associated with post-ischemic reperfusion, either spontaneous or induced by thrombolytic therapy. The molecular pathology involved in hemorrhagic conversion has yet to be fully elucidated, but considerable work has implicated enzymatic destruction of capillaries by matrix-metalloproteinases (MMP) released by invading neutrophils (Gidday et al., 2005; Justicia et al., 2003; Lorenzl et al., 2003; Romanic et al., 1998). Maladaptive activation of MMP compromises the structural integrity of capillaries. In ischemic stroke, MMP inhibitors reduce hemorrhagic conversion following thrombolytic-induced reperfusion (PMID 15459442 and 11898581). Additionally, MMP inhibitors are effective against myocardial ischemic events (Creemers et al., Circ Res. 2001 Aug. 3; 89(3):201-10).
An alternative mechanism that gives rise to PHN and post ischemic injury involves expression and activation of NCCa-ATP channels (see Simard et al., 2007). The data demonstrate that cells that express the NCCa-ATP channel following an ischemic or other injury-stimulus, later undergo oncotic (necrotic) cell death when ATP is depleted. This is shown explicitly for astrocytes (Simard et al., 2006), and in specific embodiments it also occurs with capillary endothelial cells that express the channel. It follows that if capillary endothelial cells undergo this process leading to necrotic death, capillary integrity would be lost, leading to extravasation of blood and formation of petechial hemorrhages.
However, no treatment has been reported that reduces PHN and ischemia/hypoxia associated injury with the highly selective SUR1 antagonists, glibenclamide and repaglinide, as well as with antisense-oligodeoxynucleotide (AS-ODN) directed against SUR1. It is useful that the molecular mechanisms targeted by these 3 agents—SUR1 and the SUR1-regulated NCCa-ATP channel, are characterized to further elucidate their role in PHN.
Other and further objects, features, and advantages will be apparent from the following description of the present exemplary embodiments of the invention, which are given for the purpose of disclosure.